Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications

PETROGENESIS OF ENCLAVES WITHIN THE PEGGY’S COVE MONZOGRANITE, SOUTHERN NOVA SCOTIA, CANADA

HINA IMTIAZ Trinity University Research Advisor: Dr. Benjamin Surpless

INTRODUCTION batholith, with the most voluminous period of intrusion taking place from 380 to 370 Ma The Canadian province of Nova Scotia is (Benn, et al., 1999). Within the batholith, the composed primarily of two tectonic terranes, Halifax pluton is a composite peraluminous the Avalonia and Meguma, juxtaposed by pluton, subdivided into three units (Fig. 1): the the Silurian – Devonian Acadian orogeny Harriet’s Field muscovite biotite monzogranite; (430–390 Ma; (Fig. 1)). Following regional he Halifax Peninsula leucomonzogranite; and metamorphism, polyphase deformation, and the Peggy’s Cove biotite monzogranite. cleavage formation throughout most of the Devonian (Keppie and Dallmeyer, 1995; Hicks The Peggy’s Cove monzogranite displays steep, et al., 1999), the thickened crust of the Meguma discordant contacts with the metasedimentary terrane experienced widespread intrusion by host rocks of the Meguma Group and is granitoids of the South Mountain

Meguma NOVA SCOTIA Other terrane N AVALONIA SMB 100 km TERRANE bodies SMB Peggy’s Cove MEGUMA TERRANE

Halifax Local Study Area Halifax Peninsula Harriet’s Field monzogranite Halifax Peninsula leucomonzogranite Harriet’s Field Peggy’s Cove monzogranite

Halifax pluton Halifax [ Cranberry ? Head South Mountain batholith Peggy’s Cove Meguma terrane Indian Point Atlantic Ocean Local study areas

Figure 1. Local study area in southern Nova Scotia. The Harriet’s Field, Halifax Peninsula, and Peggy’s Cove bodies are subunits within the larger South Mountain Batholith (SMB). All intrusive bodies have intruded the Meguma terrane. Cran- berry Head, Peggy’s Cove, and Indian Point localities are within the Peggy’s Cove monzogranite, adjacent to the intrusive contact with the Meguma terrane. (modified from Keppie, 2000)

255 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications thypothesized to be the earliest crystallizing higher content relative to the host rock unit of the Halifax pluton (e.g., MacDonald and with both darker (slightly more mafic) enclaves Horne, 1988). The hypothesized petrologic (4-8 cm in diameter) and megacrystic feldspars origin of the monzogranite has been determined concentrated along the boundaries of the larger (e.g., MacDonald and Horne, 1988), but little bodies and in the centers of the smaller bodies. attention has been paid to enclaves within the unit. The purpose of this study is to determine ENCLAVE PETROGRAPHY the petrogenetic origin of enclaves within the Peggy’s Cove monzogranite using field Initially, bodies of mafic appearance within relationships, thin section petrography, whole- the Peggy’s Cove monzogranite were called rock geochemistry, and microprobe analyses. enclaves to avoid genetic interpretation prior to petrographic, geochemical, and microprobe FIELD RELATIONSHIPS analyses. The enclaves have been divided into five types based on how distinct the The Peggy’s Cove monzogranite crops out petrographic textural features of each enclave along the Atlantic Ocean near St. Margaret’s were relative to the host rock as well as relative Bay, just south of Halifax (Fig. 1). Based on to other enclaves (Table 1): exposed field relationships, it is likely that the intrusive contact between the monzogranite Type 1: Fresh to moderately-assimilated and the Meguma is just offshore in the Atlantic metasedimentary xenoliths. These enclaves Ocean. While the most abundant enclave types tend to have angular to sub-angular, discrete were sampled for this study, some enclave boundaries, suggesting brittle deformation. types observed in the field were not sampled The fresh metasedimentary bodies commonly but provide important information for models exhibit features found in Meguma Group rocks, of enclave petrogenesis. These include: (1) including metamorphic textures and mineralogy enclaves composed of spherical clumps of (e.g., andalusite and sillimanite). The - nearly pure biotite (~1 cm in diameter) with ~ bearing biotite clumps described above also fall 0.5 cm garnet porphyroblasts, usually centered into a Type 1 category. The more assimilated in the enclave or clotted together on the enclave bodies consist of 40-50% subhedral biotite, margin; (2) large bodies (from ~40-50 cm to ~2- 40-50% subhedral muscovite, 3-5% anhedral 3 m in diameter) of finer grain size and slightly , 3-5% anhedral potassium feldspar, 3-5% anhedral , and trace amounts of Table 1. Enclave types. secondary (e.g., chlorite). Type Distinguishing Characteristics Possible Origin

primary sedimentary features; angular to sub-angular boundaries; and 1 xenolith metamorphic textures and mineralogy

mineralogy dominated by micas; strongly folded foliation; acicular apatite; 2 mixed/mingled and chilled margins

fine-to medium-grained; porphyritic (abundant medium-to coarse-grained 3 megacrysts of quartz and feldspars); acicular apatite; and gradational mixed/mingled magma boundaries

strong compositional banding (defined by mica-rich and quartz/feldspar 4 autolith zones); and chilled margins

fine-to medium-grained; lower mafic content than host rock; generally equal 5 autolith dimensions; and gradational boundaries

256 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications

Type 2: Fine-to medium-grained enclaves with a mineralogy dominated by micas, exhibiting strong folded foliation (Fig. 2c) and higher Plag xenocryst mafic content than the host rock. This enclave ranges from 4-5 cm in diameter and display 0.5 mm chilled margins. The biotite and muscovite define the foliation with 70-75% subhedral biotite, 15% subhedral muscovite, 4% 1 cm anhedral quartz, 3% subhedral plagioclase, 2% subhedral potassium feldspar, and trace amounts b of secondary minerals. Quartz and feldspar crystals contain acicular apatite crystals with length-to-width ratios up to 60:1 (Fig. 2d).

Plag Type 3: Fine-to medium-grained porphyritic enclaves with medium-to coarse-grained megacrysts of quartz and feldspars and bulk 0.5 mm composition (Fig. 2a) similar to the host rock. Enclaves range from 4-9 cm in diameter, c are more equant than other enclave types, and display gradational boundaries, with monotonous changes in both mineralogy and grain size across a 1-4 mm boundary zone. They consist of 30-48% subhedral plagioclase with disequilibrium textures (Fig. 2b), 5-45% anhedral quartz, 2-30% subhedral biotite, 1-35% 1 cm subhedral potassium feldspar, 1-2% subhedral muscovite, and trace amounts of secondary minerals (e.g., chlorite). Acicular apatite d crystals are present as inclusions in quartz and feldspar and exhibit length-to-width ratios up to 60:1.

Type 4: Fine-to medium-grained enclaves with strong compositional banding. These enclaves 0.4 mm range from 10–20 cm in length, with a common Figure 2. Photomicrographs of important length-to-width ratio of 4:1 or 5:1. These features within enclaves. (a) Enclave Type 3 enclaves have chilled margins, with monotonous –relatively sharp enclave–host rock boundary changes in both mineralogy and grain size with a highly altered plagioclase xenocryst inclusion within the finer-grained enclave. across a 0.5-1.5 mm boundary zone. The [Crossed polars.] (b) Enclave Type 3 – plagio- compositional banding is defined by mica-rich clase crystals with cores that exhibit disequilib- rium textures. [Backscattered image from UT (biotite >> muscovite) and quartz-feldspar- rich Austin electron microprobe facility.] (c) zones. Within each compositional band, there Enclave Type 2 – strong folded foliation, defined primarily by biotite and muscovite. is no preferred alignment of micas or feldspars [Crossed polars.] (d) Enclave Type 2 – acicular relative to the orientation of the banding, which apatite present as inclusions within quartz and is in strong contrast to Type 2 foliations. These feldspar. [Uncrossed polars.]

257 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications enclaves consist of 40-45% subhedral biotite, ENCLAVE GEOCHEMISTRY 25-40% subhedral plagioclase, 0-30% subhedral potassium feldspar, 1-11% anhedral quartz, 1- Major, minor, and trace element geochemical 2% subhedral muscovite, and 2-3% subhedral data from host rock and enclave samples are chlorite. displayed in Figure 3. These data consist of nineteen analyses of the South Mountain Type 5: Fine-to medium-grained equigranular batholith and five analyses of enclaves within granitoid enclaves with granitic texture and in the Peggy’s Cove monzogranite (two Type 3 lower mafic content than the host rock. samples and three samples from other enclaves These enclaves range from 4-6 cm in diameter, within the monzogranite). Major and minor are approximately equant in dimension, element data suggest a liquid line of descent, but and display gradational boundaries with the not all enclave samples are related to the South host rock, with monotonous changes in both 7 mineralogy and grain size across a 1.5-5.0 mm 6 boundary zone. These bodies consist of 30- MgO (%) 5 45% sub-to anhedral quartz, 20-30% subhedral 4 Enclaves biotite, 15-25% subhedral plagioclase, 4-25% SMB subhedral potassium feldspar, <1-15% subhedral 3 chlorite, and <1% subhedral muscovite. 2 The significance of acicular apatite 1 Wyllie and others (1962) experimentally 0 determined that the habit of apatite crystals 3 varies with the conditions of formation. Factors 2.5 TiO2 (%) include cooling rate, composition, and vapor or 2 liquid present. Their experiments showed that crystals precipitated from liquid during a quench 1.5 Enclaves process form acicular prisms that are greatly SMB elongated parallel to the c-axis. The occurrence 1 of acicular apatite is caused by rapid cooling 0.5 rates and resulting precipitation rather than the crystal’s chemical environment (Wyllie et al., 0 1962). Acicular apatite has never been observed 60 Ni (ppm) in metamorphic rocks (Wyllie et al., 1962). 50 Therefore, the enclaves which bear acicular 40 Enclaves apatite (Types 2 and 3) likely had a magmatic SMB origin and were involved in a rapid cooling 30 event. 20 10 0 50 55 60 65 70 75 -10 SiO2 (%) Figure 3. Major, minor, and trace element data from the South Mountain batholith (SMB) and enclaves within the Peggy’s Cove monzogranite. Yellow trends indicate liquid lines of descent based on high-silica enclave and SMB sample data. Not all enclaves can be explained by simple models of fractional crystallization.

258 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications both the enclaves and host rock suggest a Mountain batholith and/or the other enclaves by significant change in environment prior to rim a simple fractional crystallization model (Fig. crystallization, and the sodic rims on plagioclase 3). Trace element data also define a liquid line crystals in the host rock suggest that final of descent, but as with major element data, not crystallization of the host rock occurred after all enclave compositions follow these trends. final enclave crystallization. Geochemical data provide evidence that samples from the South Mountain batholith host rocks are relatively homogeneous, and while some DISCUSSION enclaves within the batholith are geochemically similar to the host rock, another model must The data acquired from field observations and be used to explain the petrogenesis of some petrographic, geochemical, and microprobe enclaves. analyses allow development of a model that can both explain the origins of enclaves observed only in the field as well as those sampled for PLAGIOCLASE CHEMISTRY this study. Type 1 enclaves are hypothesized to be mildly to moderately assimilated xenoliths Four enclave samples were selected for electron from the Meguma terrane and Types 4 and 5 microprobe analysis of plagioclase feldspar. enclaves are thought to be autoliths produced Plagioclase crystals both inside the enclaves by magmatic processes entirely within the and in the host rock exhibit zoning and were monzogranitic magma. However, Types 2, 3, analyzed from core to rim for sodium and and the larger, enclave-bearing bodies described calcium content. Most crystals within the in the Field Relationships section require a more enclaves have both altered cores, which suggests detailed explanation. disequilibrium (Hibbard, 1994), and at least one fresh, well-defined growth rim (Fig. 2b). In the A model that may explain the origins of all host rock, most plagioclase crystals exhibit two observed enclaves is presented in Figure 4. In or more fresh growth rims around altered cores, this model, there is not a large compositional but crystals with oscillatory zoning were also difference between the “mafic” and common. . The felsic magmas supplying the active chamber remain fairly constant in Most plagioclase within the host rock varied in composition, and the petrologic differences composition, with contents ranging from An03 to within the monzogranite magma are primarily An51, without a well-defined core – rim pattern. controlled by biotite crystal fractionation Where core – rim trends were present, the (MacDonald and Horne, 1988). outer rim showed a significant jump in sodium content (from An40 to An08) that wasn’t observed Field observations indicate that there are both in any enclave. Plagioclase crystals within the enclaves present as inclusions in larger, more enclaves didn’t exhibit either normal or reverse mafic, finer-grained bodies as well as enclaves zoning where multiple rims were present and isolated within the monzogranite (the main displayed contents from An15 to An58, but the focus of this study). The larger, ~40-50 cm to extensive alteration of cores didn’t permit their ~2-3 meter fine-grained bodies are thought to be analysis. hybrid magma bodies which have resulted from some period of magmatic evolution within a With the exception of the outermost rims segregated chamber or deeper area of the larger of plagioclase in the host rock, there was chamber that is intermittently fed by a slightly little difference between anorthite contents more mafic source, resulting in a hybrid, mixed in the enclave rims versus in the host rock magma with enclaves distributed within it cores and rims. However, the disequilibrium (Figure 4). textures observed in plagioclase cores within

259 Imtiaz, H. 2007. 20th Annual Keck Symposium; http://keck.wooster.edu/publications

Later pulses of the more felsic source magma are thought to have entrained this hybrid, enclave-bearing magma and brought it into the monzogranite-dominated magma. Other surges of “mafic” magma are thought to have bypassed the deeper hybrid magma and are injected directly into the monzogranite. The temperature Second surge of “mafic” magma contrast between the “mafic” magma and the felsic monzogranite isn’t great enough to produce true quenched margins on the enclaves, but the mingled/mixed enclaves (Types 2 and 3) exhibit features that suggest a significant drop in temperature (acicular apatite), a change in composition of surrounding magma (disequilibrium textures in the plagioclase), and mixing along the enclave-host rock interface (plagioclase xenocrysts within the enclave). “mafic” Injectionmagma of Further study of both enclaves and enclave – host rock relationships is required to test the

validity of this model. source felsic magma Second surge of

Felsic magma Felsic REFERENCES

Benn, K., et al., 1999, Geophysical and KEY structural signatures of syntectonic Meguma xenolith (Type 1) batholith construction: the South “Mingled” enclave (Types 2 & 3) Mountain Batholith, Meguma Terrane, “Mingled” enclave (within hybrid magma) Nova Scotia; Geophysical Journal Autolith (Types 4 & 5) International, v. 136, p. 144-158. Hybrid magma body Monzogranite magma Hibbard, M.J., 1994, Petrography to Hybrid magma Petrogenesis: Engelwood Cliffs, New Meguma terrane Jersey, Prentice Hall, 608. Magma flow path Hicks, R. J., Jamieson, R. A., and Reynolds, P. Figure 4. Hypothetical model for petrogenesis of 40 enclaves within the Peggy’s Cove monzogranite. H., 1999, Detrital and metamorphic Ar/ 39 Enclave data suggest at least two sources of magma Ar ages from muscovite and whole-rock in the Peggy’s Cove system. The presence of samples, Meguma Supergroup, southern enclaves both as isolated bodies within the monzo- Nova Scotia: Canadian Journal of Earth (the focus of this study) and as inclusions Sciences, v. 36, p. 23-32. within larger bodies of fine-grained, slightly more mafic bodies suggests a two-stage system of mixing and differentiation. See text for full explanation of Keppie, J. D., 2000, Geologic Map of the the model. (extensively modified from Hibbard, Province of Nova Scotia, Nova Scotia 1995) Department of Natural Resources: Minerals and Energy Branch, scale 1:500,000.

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Keppie, J. D., and Dallmeyer, R. D., 1995, Late Paleozoic collision, delamination, short- lived magmatism, and rapid denudation in the Meguma terrane (Nova Scotia, Canada): Constraints from 40Ar/39Ar isotopic data: Canadian Journal of Earth Sciences, v. 32, p. 644-659.

MacDonald, M. A. and Horne, R. J., 1988, Petrology of the zoned, peraluminous Halifax Pluton, south-central Nova Scotia: Maritime Sediments and Atlantic Geology, v. 24, p. 33-45.

Wyllie, P.J., Cox, K.G., and Biggar, G.M., 1962, The habitat of apatite in synthetic systems and igneous rocks: Journal of Petrology, v. 3, p. 238-243.

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